The invention relates to new polymer materials having non-linear optical properties and to processes for their preparation.
Non-linear optics deals with the interaction of electromagnetic fields in various media and the consequent formation of new fields with altered properties. Materials having non-linear optical properties possess a field strength-dependent dielectric susceptibility of the 2nd order which has a series of dispersive processes as a consequence: the frequency doubling (second harmonic generation=SHG) permits the production of light of half the wavelength compared with the irradiating light; the electrooptical effect (Pockels effect) makes possible a change in the refractive index with applied electrical field; methods of sum and difference frequency mixing and frequency splitting allow the continuous tuning of laser light.
A large number of technological applications result from the previously mentioned effects. Electrooptical switches, frequency and intensity control in laser technology, holography and the areas of information processing and integrated optics represent areas of use for materials having non-linear optical properties of the 2nd order.
Materials having electrical susceptibility functions of the 3rd order are suitable for the preparation of purely optical switches and thus as waveguides for construction of purely optical computers.
In order to be suitable for application in the area of non-linear optics of the 2nd order, materials of this type have to fulfil a number of requirements.
In addition to a non-centrosymmetrical molecular arrangement in the crystal, technological utility calls for as high as possible a value for the dielectric susceptibility X.sup.( 2).
A number of inorganic substances such as, for example, potassium dihydrogen phosphate or lithium niobate show non-linear optical properties. However, all these compounds are affected by all sorts of disadvantages. In addition to insufficient values for the dielectric susceptibility of the second order, inorganic compounds frequently lack sufficient photostability on treatment with high light intensities, or they can only be prepared and processed with difficulty.
Organic compounds of the nitroaniline type are known from Garito et al., Laser Focus 18 (1982) and EP-0,091,838. However, their relatively good values for photochemical stability and dielectric susceptibility of the second order go along with poor crystallizability and deficient mechanical stability. In particular, the preparation of thin layers, such as are required for integrated optics, is unsuccessful with these materials.
Polymers are characterized by high mechanical resistance capabilities and good chemical stability. Molecules having non-linear optical properties which are attached to the polymer framework or dissolved in the polymers should therefore have advantageous values for dielectric susceptibility in the non-centrosymmetrical environment.
Polymers having non-linearities of the second order can be prepared by applying an external field to a film heated above the glass temperature and doped with randomly orientated molecules. This leads to a polarization of the included molecules which confers an anisotropy to the polymer medium after solidification thereof. Polymers having non-linear optical properties prepared in this manner, in which p,p'-dimethylaminonitrostilbene is used as the host molecule, were described by Meredith et al., Macromolecules 15 (1982) 1385.
Shibaev et al., Polymer Communications 24 (1983) 364, report the field-induced alignment of liquid crystal polymers with mesogenic side groups.
U.S. Pat. No. 4,412,059 discloses a polymer material having cholesteric mesophases which are accessible by means of electrical or magnetic fields of a controlled alignment. Finally, fully aromatic, thermotropic, liquid crystal polymers are known from EP-0,172,012, the non-linear optical properties of which can likewise be produced by external fields.
A further method for the production of polymer materials having non-linear optical properties consists in the polymerization of monomers which are already ordered and have a non-centrosymmetric orientation, the state of order of the system being essentially maintained during the polymerization. Suitable monomers for this technique are to be taken, for example, from EP-0,021,695.
The materials described previously still show unsatisfactory non-linear optical properties which are expressed in particular in deficient values for the dielectric susceptibility and the photochemical stability.